1. Field of the Invention
The present invention relates to laser systems. More specifically, the present invention relates to nonlinear optics and high power lasers.
2. Description of the Related Art
Nonlinearity cells are commonly used in applications such as phase conjugate mirrors to generate real-time holograms. A nonlinearity cell typically includes a window containing one or more layers of a nonlinear optical medium whose optical properties (usually its refractive index) change in response to the presence of light. When an interference pattern formed by two beams of light is applied to the nonlinear medium, the interference pattern is reproduced in the medium as a pattern of high and low refractive index, effectively creating a diffraction grating that is a real-time hologram of the optical interference pattern. The nonlinear medium tracks changes in the interference pattern formed by the two beams such that as the interference pattern changes, the diffraction pattern in the medium changes as well.
A loop phase conjugate mirror (LPCM) system typically uses a liquid thermal nonlinearity cell in a four-wave mixing configuration to create a holographic mirror. Four-wave mixing involves the use of two counter propagating pump beams and a nonlinear medium. The beams propagate through the medium along with a signal beam of which a phase conjugate beam is to be generated. The signal beam propagates through the medium at an angle relative to at least one of the pump beams and thereby creates an interference pattern. The remaining pump beam reads the interference pattern as a hologram producing a phase conjugate return of the signal beam.
The absorption of light makes the thermal nonlinearity cell operational. The same absorption process, however, heats the nonlinear medium; that heat must be removed to avoid optical damage. The cell must be designed such that the absorption and attendant temperature contrast are adequate to produce the desired effect; but: the maximum temperature anywhere within the liquid medium must remain below the boiling point of the liquid. This can be a problem for high power applications.
Conventional thermal nonlinearity cells typically include a thin layer of slightly absorptive liquid (usually an organic solvent) placed between a transparent window and a highly reflective mirror, which may be cooled to provide a constant temperature heatsink. However, a single thin layer thermal cell usually cannot provide good phase conjugation fidelity due to the insufficient selectivity of a conjugated wave by a thin hologram. In addition, a one-layer hologram is not well projected to high power scaling because its capability in sustaining high laser power and thermal load in just one layer of absorbing liquid could be limited.
Other thermal cell designs for use in LPCM systems in particular have used one or more thin layers of liquid sandwiched between a series of optically transparent windows. The liquid is forced to flow to remove the generated heat by moving the heated liquid away from the laser interaction region. This technique, however, requires a mechanical means for providing the liquid flow, such as pumps, etc., which may be costly to construct and operate. In addition, the flow velocity of the liquid should be carefully controlled such that it has a capability to remove the overall heat but not destroy the positive effect of local heating that results in the hologram fringes recording. The hologram may be washed out if the liquid is moved too quickly. For high average power operation, the flow velocity and the pressure drop could be too high to be able to project to the desired power scalability.
Another prior approach described in a patent application entitled CONDUCTIVELY COOLED LIQUID THERMAL NONLINEARITY CELL FOR PHASE CONJUGATION AND METHOD, Ser. No. 10/866,201 filed Jun. 10, 2004 by A. A. Betin, N. P. Davis and J. J. Ichkhan, the teachings of which are hereby incorporated herein by reference, used a conductively cooled cell comprised of a multi-layer transmission cell with thermally conductive windows that removed heat to peripheral heat sinks. Practical implementation of this approach relies on good quality optics and multiple surfaces with complex high transmission and/or high reflective coatings, which may lead to excessive losses and compatibility problems with the nonlinear liquid. Another problem in practical implementation comes from the fact that the liquid layers should be very thin for rapid heat transfer, but should have enough absorption for the thermal nonlinearity mechanism to take place. A high absorption coefficient in liquid is typically achieved by adding special impurities into the liquid that are absorptive at the laser-wavelength, thus creating a problem with the choice of materials and compatibility with the liquid and its stability.
Hence, a need exists in the art for an improved nonlinearity cell design that offers higher performance, robustness, and easier maintenance with the capability of power and fidelity scalability for high power applications.